Traditional silicon-based solar cells are completely opaque, which works for solar farms and rooftops but defeats the purpose of windows. A transparent solar cell will change the rules of the game.
Now, in an effort to bring transparent solar cells to home windows, researchers at the University of Michigan have developed a technique to make their highly efficient and semitransparent solar cells at scale.
Organic solar cells, where the light absorber is a type of plastic, can be transparent. But these cells have lagged behind their silicon-based cousins for energy generation purposes due to engineering challenges such as low efficiency and short life, but recent work on Forrest’s lab achieved a record efficiency of 10% and estimated lives of up to 30 years.
A key challenge in making transparent solar cells is making the micron-scale electrical connections between the individual cells that make up the solar module. Conventional methods that use lasers to shape cells can easily damage organic light absorbers.
However, the researchers have developed a multistep peel-off patterning method that achieves micron-scale resolution. They deposited thin films and patterned them into extremely thin strips, and deposited organic and metallic layers. The team then peeled off the strips, creating excellent electrical interconnections between the cells.
The team connected eight semitransparent solar cells, each 4 cm x 0.4 cm and separated by 200µm-wide interconnections, to create a 13-square-centimeter module. The power conversion efficiency of 7.3% is approximately 10% lower than that of individual solar cells in the module. This small efficiency loss does not increase with module size; therefore, similar efficiency is expected for the meter-scale panel as well.
With transparency close to 50% and a green tint, the cells are suitable for use in commercial windows. Higher transparencies that are likely to be preferred for the residential market can easily be achieved with this same technology.
“Now is the time to involve industry to turn this technology into affordable applications,” said Xinjing Huang, UM doctoral student in applied physics and the first author of the published research.
Eventually, the flexible solar cell panel will be placed between the two window panels. The goal for these energy-generating window films is to be about 50% transparent with 10%-15% efficiency. The technique can also be generalized to other organic electronic devices. Researchers have already applied it to OLEDs for white light.